Electrostatic printing



July 10, 1962 E. F. MAYER ELECTROSTATIC PRINTING 2 Sheets-Sheet 1 Filed Jan. 26, 1955 INVENTOR. EDWARD F. MAYER BY LAWRENCE I. FIELD ATTORNEY July 10, 1962 Filed Jan. 26, 1955 E. F. MAYER 3,043,684

ELECTROSTATIC PRINTING 2 Sheets-Sheet 2 I 9 I I I 8l l4 INVENTOR.

EDWARD F. MAYER BY LAWRENCE I. FIELD ATTORNEY United States PatentOfifice r sauna; Patented July 10, 1962;

3,043,684 ELECTROSTATIC PRINTING Edward F. Mayer, Cleveland, Ohio, assignor, by mesne assignments, to General Dynamics Corporation, San Diego, Calif a corporation of Delaware Filed Jan. 26, 1955, Ser. No. 484,215 8 Claims. (Cl. 96-4) This invention relates to electrostatic printing and includes a novel electrostatic printing device as well as a novel method of utilizing such a device in the production of an ultimate visual reproduction of an electrostatically produced image.

There have been developed heretofore electrophotographic devices wherein a visual image is converted to an electrostatic image and the resulting electrostatic image is thereafter made visible by the deposition of powder in a pattern conforming to the electrostatic image. Devices in which such a method is described include those patented by Chester F. Carlson, in U.S. Patents 2,221,776; 2,277,- 013, and 2,297,691, among others. Another such device is described in my copending application for Letters Patent Serial No. 380,285, of which this application is a continuation in part. I have now discovered a novel method and apparatus in which certain deficiencies and disadvantages of the prior art apparatus are overcome. By employing the teachings of my invention, it is now possible to receive and to print images at a rate hitherto unobtainable. To emphasize the diiferences between my invention and the prior art, a brief resume of each of the aforementioned patents and copending applications follows.

Carlson 2,221,776 describes a method and apparatus utilizing a photosensitive element consisting of a thin transparent metal film deposited on a sheet of transparent material and a photoelectric layer. The photosensitive layer disclosed by Carlson is preferably conductive and is composed of a photoemissive material. In operation, the photosensitive element is connected to one terminal of an electricbattery and while so connected is exposed to a suitable source of illumination whereby an image is focused thereon. This image is reproduced in the form of a latent electrostatic image on a translucent transparent material of high insulating value which is positioned between the photoemissive element and ametal plate connected to the other pole of the electric battery, In such an apparatus, it will be understood that the latent electrostatic image is produced by the photoemissive material which, under the influence of light, projects electrons into the space between the insulating layer and the photosensitive element. These electrons are then acted upon by the electric field and drawn to the, surface of the insulating layer. The latent electrostatic image is subsequently de- 23 cured to one another in the form of a permanent unitary assembly. The latent electrostatic image is obtained in one of the insulating layers by impressing a potential across the photoconductive element which forms a sort of'condenser in a circuit in which one conductive layer of the photoconductive element is connected to one pole of abattery and the other conductive layer of the photoconductive element is connected, through a switch to the other pole. With the layers so connected, i.e. with the full potential of the battery across the photoconductive element, the photoconductive element is exposed so that the article whose picture is to be reproduced is imaged thereon. The light causes the illuminated portions of the insulating photoconductive layer to become conductive and permits electrons to migrate through the photoconductive layer' towardthe conductive layer adjacent the light where they are trapped. As described in the patent, the maximum charge occurs with the simultaneous application of voltage and light. The charge magnitude as further described in the patent is proportional to the light exposure and the applied potential. I have attempted to utilize this latent electrostatic image for the production therefrom of a visible image, but have found that the subsequent charging of either of the conductive layers in the development of such a visible image destroys the electrostatic image.

In Carlson 2,297,691 a method and apparatus are de- I scribed employing a photoc'onductive plate comprising a thin layer of photoconductive insulating material bonded to a metal plate. According to the description, the plate may be charged in either of two equivalent ways. In one,

veloped by contacting the insulating sheet material with a v suspension of afine powder or dust or by contacting the sheet with a fine atomized mist of ink. At the speeds at which I desire to operate, I have found that both the means for forming the latent image and the means for developing the image described in Carlson 2,221,776, are entirely inadequate.

In Carlson 2,277,013, another method is described for obtaining a latent electrostatic image. In this patent the photoconductive element comprises a backing layer formed of a thin layer of metal orother highly conductive material covered by a thin layer of a solid dielectric material such as Al O or lacquer, having a 'high insulating value and free from any tendency to become a conductor of electricity under ordinary illumination, upon which there is directly disposed a layer of asubstance'which is described as a photoconductive insulating material and finally a thin transparent conductive layer of a metal or These four layers are sethe photoconductive insulating layer is charged by rubbing it vigorously with a soft material such as a cotton or silk handkerchief, whereby an electrostatic charge'is developed thereon by friction. In an alternative method, the plate is connected to one pole of an electric battery the other pole of which is connected to an. opposed plate spaced from thephotoconductive insulating layer by an air gap. In this instance, the plate is charged only upon exposure to a source of illumination. In both instances the charged plate is exposed to animage. illuminated thereon whereupon the charge in the illuminated area is permitted to leak off to the conducting backing layer. One disadvantage inherent in this patent is that the method requires that the photosensitive element must be charged prior to the projection thereon of the desired image. A further disadvantage in this method is the very stringent requireriorating for a sufiicient time to permit'the subsequent steps of exposureand development.

In Carlson patents 2,357,809 and 2,551,582 the electrophotogra-phic principles illustrated in the earlier issued Carlson patents are embodied in apparatus in which they are not substantially modified. t

In my copending application Serial No. 380,285, filed September 15, 1953, I have disclosed that if the single element of the aforementioned Carlson Patent 2,277,013 is divided'into two separate elements it is possible to obtain a visual image from the electrostatic image produced through the cooperation of these two elements. Thus, the electrophotographic device of my copending application comprises a photoconductive element and an adjoining but separable electrostatic image-forming element. The

some cases, the electrically conductive material contiguous to the photoconductive material of the first-mentioned element may be'opaque, in which case both components of the electrostatic image-forming element are transparent. In either case, these two elements are so disposed with respect to one another thatthe layer of protoconductive material of one element adjoints the layer of insulating material of the other element. A source of electric potential is provided which interconnects the transparent layer of electrically conductive material of one element with the electrically conductive material in the other element so as to provide the latter with a positive potential with respective to the former, and light means are also provided for directing a light image of the object toward and through the transparent layer of electrically conductive material in the direction of the contiguous layer of photoconductive material.

The method of producing a visual reproduction of a visual object with the electrophotographic device-of my copending application Serial No. 380,285 comprises directing alight image of the object through the transparent layer-of electrically conductive material and thence through the contiguous high resistance photoconductive layer to the adjacent but separable layer of electrically insulating material backed by the supporting metal layer. A positive electric potential (or negative electric potential, if a negative electrical image is desired) is applied to the supporting metal layer with respect to the transparentqlayer of electrically conductive material, thus static reproduction of the light image. The surface of the insulating material bearing theelectrostatic reproduction' of the image, is thereafter separated from the previously adjoining photoconductive material. This electrostatic image comprises charges which are inversely proportional to the illumination received by each incremental .area. The electrostatic image is developed by application .of a. dispersion. of-powdered carbon with the resulting formation. on the surface of the separated insulating material. of a carbon deposit comprising a visual reproduction. of the aforesaid lightimage.

It-will be seen that the device of. my.v copending applicationis entirely difierent from that disclosed in Carlson 2,277,013, both structurally and. in themanner in whichitis employed. In my earlier application, the electrostatic charge. image formed on the image-forming layer is developed directlyinto a visual image by the application theretoof a material such as finely divided carbon. 'In the Carlsondevice, however, the electrostatic image formed'on the surface of the insulating layer remains trapped,=(the exact and very apt word used by Carlson;on page-3, col. 2,-line-59), at the interface between the dielectric layer; and the adjacent photoconductive insulating layer; s This trappedelectrostatic image is not developed-directly into a'visual image but, instead, is scanned with ,abeam of light so that the charge voneach incremental area ofthe interface illuminated by .Serial -No. 380,285 has beenfurtherinvestigatedin an eflort to adapt it to electrophotographic reproductionin high speed-printing. My earlier application disclosed a method andapparatus comprising essentially a photoconductive element-on which the desired picture is focused, an image-forming element, means. to charge the imageforrning element, means to develop the image thereon and means to transfer the image-to a permanentrecord me- .dium such as a sheet of paper or an endless web of paper.

Means were also provided for cleaning the image-forming'drum and for discharging same. r 7 In my earlier application the formation of-the desired electrostatic 'image was effected-at the points of contest f; he, cha r m a d drum carrying; the

establishing on the layer of insulating material an electroing the coating layer on the charging drum, that is by choosing a suitable material of reduced resistivity for the outermost layer of the charging drum or other charging means, it becomes possible to impart a charge to the photoconductive surface by leakage through the charging drum instead of leakage through the photoconductor.

One object of this invention is to provide a method and apparatus for receiving and printing images by electrophotographic means at high speeds.

A further object of this invention is to provide an apparatus in which wear on the photoconductive material employed in the apparatus is reduced to a minimum.

A further object of this invention is to provide an ap paratus for receiving and printing images by electrophotographic means which is characterized by simplicity, dependability, ruggedness and low cost. I

These and other objects will become readily apparent to those skilled, in the art by reference to the following specification and drawings in which:

FIGURE 1 is a view'of a preferred embodiment of my electrophotographic appartaus seen in section;

FIGURE 2 is a similar view of a slightly different apparatus in lwhichthe charging drum has been modified;

FIGURE 3 is a similar. view of the apparatus. of FIG- URE 1 in which the means for developing a visible image have been separated from the photoconductive ma terial;

FIGURES'4 and 5 represent in. section, two methods for the charging of a photoconductive element when in the form of a flat plate.

The electrophotographic device shown in FIGURE 1 is representative of the'invention. In this device the photoconductive element and the electrostatic image-forming element havethe form of rotating drums 6 and 7. The drum 6 is of a transparent material '8 such as a glass cylinder... This, cylinder is coated on its outer surface with a transparent conductive layer 9 such as a thin layer of tin oxide, or of evaporated gold withbismuth additions on the glass, or other conventional metallic conductors which are transparent in thin layers. The. cylinder may alsobe fonmedof any transparent plastic with the transparent conductive coating upon its outer surfaces. A third alternative construction comprises a conductor chemically depositedupon the surface of a glass or plastic cylinder. The primary requisite of these materials is that they be of fairly low resistance, for example to 1000 ohms per'square surface resistance, and that they be moderately transparentto visible'light, that is having a transparency of about 50% to white light. These transparency requirements are arbitrary, and through the use of a higher or lower intensity light they may be greater or less, as the case may be. Over the aforementioned conductive coating 9 upon the cylinder there is formed a layer of a high resistance photoconductor 10. This photoconductivelayer should have resistivity in the dark of 10 ohm-cm. or greater, and a resistivity when exposedto the maximum light intensity of 10 ohm-cm. or less. Again, these resistivity values are arbitrary and may be varied as other factors are altered to meet various operating conditions. Selenium, arsenic selenide, zinc-cadmium sulfide, and many other known .materials which have the aforementioned properties are suitable. 'Ifhese photoconductive layers'should be continuous and are advantageously deposited by vacuum deposition.

The second drum 7 is made of metal 11 coated with a material 12. This material is selected so that as the drums rotate in contact with a potential 15 applied, charge may leak through from the metal core 11 to the surface of the photoconductor 10 at the desired rate. The maximum speed of operation desired and the maximum charge desired on the photoconductor surface together with the capacity between the line of contact and the conducting layer of drum 6 all determine the specific value of resistance required. I have found that the characteristic required of material 12 is that the resistance from a point on the surface to the core 11 lie within the range of 10" and 10 ohms. These values are for a specific set of operating speeds and may be raised or lowered as the speed is increased or decreased. Suitable materials :for layer 12 include cellulose acetate and other transparent cellulosic resins or other semi-conducting organic materials such as conventional semi-conducting rubbers.

The two drums 6 and 7 are positioned so as to rotate in opposite directions in contact with each other along one element of each cylinder. In the center of drum 6 there is placed a light source 13 which possesses illumination characteristics similar to the sensitivity characteristics of the photoconductive material. A (silver halide) positive 114 to be reproduced is placed in contact with the interior of the drum 6 between the light source 13 and the transparent conductive layer 9.

Between the positive 14 to be reproduced and the light source 13, there is positioned a shield 26 to restrict the illumination to a narrow area of the photoconductive layer 1 0. Shield 26 is maintained stationary while the drum rotates. In lieu of a directly positioned positive, any optical image focused upon the point of contact of the two drums 6 and 7 may be used. This optical image may be of the continuous tone type or simply a line negative.

Instead of an internally positioned source of illumination :13, the illumination and image formationon photoconductive layer 10 may be produced by means of an externally positioned image-forming device which may be preferably positioned so as to form the image before the point of contact of the drums.

As the two drums 6 and 7 rotate in opposite directions, their line of contact progresses around their surfaces. A potential source 15 of approximately 900 to 1200 volts D.C. is applied between the metal drum 11 and the conductive layer 9. This permits charge from the potential source 15 to leak through the high resistance of the layer '12 to the surface of the photoconductor 10. A small capacitor is formed continuously between the interface of roll 6 and 7 and'the conducting layer 9. Charge leaking through the high resistance layer 12 tends to charge this capacitor to a value determined by the resistance of the layer 12, the speed at which the drums are rotating and the size of the aforementioned capacitance. The charge accepted by photoconductive layer :10 will be determined therefore, by the illumination falling on said layer. Thus, there is built up upon the photoconductive layer 10 a charge configuration inversely proportional to the light and shadow pattern of each corresponding area of the image to be reproduced. By suitably positioning shield 26, the image may be formed on photoconductive layer 10 either before or after the line of contact of drums 6 and 7 or at the line of contact if desired. Similarly, when an external illumination and externally positioned image-forming means is employed, the image may be formed at a point immediately preceding or immediately subsequent to or at the line of contact of the two drums.

The operation of the device under conditions of exposure prior to the line of contact requires that the device be operating at a speed great enough to cause the time interval between the exposure of any point on the surface :10 and the contact of exposed point on the surface 10 with material '12 covering the charging drum 7 to be small compared to the lifetime of the carriers developed by the light in the photoconductive layer It When these conditions are fulfilled, the charge leaking through the high resistance layer 12 performs the dual function of discharging the carriers developed in the photoconductive layer in the illuminated areas and developing a potential in the non-illuminated areas.

In the case where the illumination is subsequent to the contacting of the layer with the drum 7, the charge potential leaked through the layer 12 to the surface of the photoconductor 10 at the time of contact produces a uniform potential on the surface of the photoconductor it). Subsequent exposure of portions of the photoconductor by projection thereon of an image reduces the resistance of the illuminated areas and discharges a portion of this charge on the surface of photoconductor 10. The resultant electrostatic image on photoconductor 10 is therefore inversely proportional to the light intensity falling of the photoconductor 10.

The development of a visual image from any of the aforementioned electrostatic images on the photoconductive layer 16 of the drum 6 may be effected-by any of the conventional procedures described and used heretofore in this art. For example, a dry powder which is electrostatically charged and suspended in air is attracted to the charged areas constituting the electrostatic image on the photoconductive layer It A common material used as this powder is carbon black. Thus, bringing a suspension of charged carbon black in air into contact with the electrostatic image-bearing portion of the photoconductive layer'ltt, produces on the photoconductive layer it a carbon deposition inversely proportional to the illumination of the original picture. This, therefore, is a positive image of the original picture or light image. The carbon black suspension is brought into contact with the surface of the photoconductor layer 10 by a means 22 through which the electrostatic image passes soon after leaving the line of contact with the insulating layer 12 of drum 7. The resulting carbon black image is subsequently removed from the surface of the photoconductive layer 10 in a later zone in the peripheral travel of this image during continued rotation of the drum 6. Removal of the carbon black image may be readi y effected by means of a roller 16 bringing an adhesivecoated surface, or even an untreated surface, of a moving sheet of paper 17 into transient but relatively non-moving contact with the moving surface of the photoconductive layer it). The image is thus transferred to the paper, and the surface of the layer ltlis subsequently reconditioned by its contact with a conventional cleaning means 18 to prepare the surface of the layer 10 for its next contact with the layer 12 of the drum 7.

While the use of a suspension of dry carbon black in air is suitablefor low speed prior art purposes and may be employed in my apparatus when operated at such speeds, I have found that greatly increased speed of operation together with a simplification in the development processes may be obtained by use of a liquid dispersion of carbon black in place of a suspension of carbon black in air.

The suspension may be conveniently applied by means of a metering roller 28 (FIGURE 3) rotatably supported in means 22 so that it contacts roller 6 at a line of tangency. The roller applies the carbon black suspension to photoconductive layer It). The carbon black precipitates thereon in proportion to the amount of charge present on the surface of the photoconductive layer 10, whereby a positive of the inversely impressed light image is produced on the surface. The resulting carbon black image may be subsequently removed by transfer roller 16 carrying an endless web of paper 17, as shown in URE 1. device comprises a drum 7 substantially the same as that the photoconductor for further use.

In the modification shown in FIGURE 2, the drum 6 is the same as in FIGURE 1 but the drum 7' is ditferent in that the central metal core 11 is smaller than that of FIGURE 1 and a resilient rubber layer 20 is positioned about its periphery. Any commercial type of rubber of hardness sufiicient to establish intimate contact of the outer insulating layer 12 with the surface of the photoconductive layer 10 may be used for this purpose. Coated upon the exterior of the rubber layer 20 is a conductive layer 21. This conductive layer need not be transparent nor have any properties other than low electrical resistance. Thus, the conductive layer 21 may be made of an electrodeposited metallic layer, or of an evaporated metallic layer or a chemically deposited metallic layer. It may be a metal or a conventional semiconductor of very low electrical resistance. In any event it should be thin enough to bend readily with the surface of the underlying rubber layer and adhere well to the rubber surface. Over this conductive layer '21 is a thin insulating layer 12 made up of a material with the desired I electrical properties as previously mentioned. The drum 7' thus produced is then placed in the position of drum 7 as shown in FIGURE 1 and the operation of the resulting device is identical with that described in connection with FIGURE 1. The resilient rubber backing for the conductive metal layer 21 permits the insulating layer 12 to make more uniform contact with the photoconductive layer 10 in spite of minor irregularities in the symmetry of these two adjoining layers.

The modification of the electrophotographic device shown in FIGURE 3 embodies a photoconductive drum 6 which is identical with the corresponding drum in FIG- The second element of the electrophotographic shown in FIGURE 1. In order to spare the surface of the 'drum 6 from the slightly abrasive action of the developer powder which is applied to and removed from the drum 6 in the embodiments of FIGURES 1 and 2, a third drum 23 is positioned in rolling contact with the drum 6. The drum 23, like drum 7, is composed of an inner metal component 41 and an outer insulating layer 42; The electrostatic image established on the surface of the photoconductive layer 10 of the drum 6 is transferred to the corresponding surface of the drum 23 along the line of contact between these two drums. The

electrostatic image on the drum 23 is then developed through the medium of the coating means 22 and the transfer means 16, and the surface of the drum is prepared for further use by cleaning means 18, such as that described in connection with FIGURE 1.

In lieu of drum-shaped or cylindrical elements, one or bothof these elements may be in the form of a flat bed. For example, as shown in FIGURE 4, the photoconductive element corresponding to the drum 6 in FIG- URE- 1' comprises a glass plate or other transparent backing material 8, a transparent conductive layer 9', and a high resistance photoconductive layer 10'. These layers are similar to those more fully described in conjunction with FIGURE 1. Placed immediately below this platelike element is a positive '14 of the image to be reproduced. Below this and positioned so as to illuminate. the photoconductive layer through this positive image is placed a light source 13'. Positioned so as to roll across the surface of this photoconductor is a drum-shaped element constructed the same as drum 7' in FIGURE 2. Electrical contact is established between the transparent conductive coating 9' on the plate element and the conductive cylinder 21 through a lead 27 connecting the flexible metal layer. 21 to metal core 11 and a battery .15 'isconnected between conducting layers 21 and 9'. As the roller progresses across the flatplate, a charged image is developed on the surface of the photoconductive layer 10' of the plate. This image is then moved to a developing means not shown) and the developed visual image is removed on a permanent record sheet. Upon completion of one traverse of the photoconductive plate element the charging drum element 7 stops, is lifted 0E and is returned to its starting position by any conventional mechanism.

The modification shown in FIGURE 5 is one in which both of the elements are in the form of a fiat plate. In this modification the entire electrostatic charge image is developed at one time as distinguished from the line-ata-time development of the electrostatic image on the photoconductive surface in the modifications shown in FIGURES 1 through 4. The plate-shaped photoconductive element of the device shown in FIGURE 5 is the same as that shown in FIGURE 4. The second fiat element of the device of FIGURE 5 comprises a rigid metal backing plate 24, a resilient layer 25 of rubber or similar composition, a conducting layer '21 and an insulating layer 12'. This second plate-shaped element is brought into contact with the photoconductive surface of the first plate element. A positive 14 of the object to be reproduced is placed beneath the bottom surface of the glass layer 8 and illuminated from a light source 13'. Simultaneous with this a potential is applied between the metallic conducting layer 21' and the transparent conducting layer 9. With these conditions maintained, a charge is built up on the surface of the insulating layer 12 and the photoconductive surface 10' in non-illuminatcd areas. In illuminated areas little or nocharge will be developed. These two plate elements are then separated while both the illumination and the potential are maintained. This produces a charge configuration on the surface of the photoconductive layer 10' which is inversely proportional to the transparency of the original image. This is then transported to an adjacent area, and developing means similar to those used to render this image visible in the procedures previously described herein are employed. The image is then removed by conventional means. This process is adaptable to flatbed printing processes and can be incorporated into a standard printing operation.

As mentioned hereinbefore, an improvement in the procedure for developing the electrostatic image into a visual image consists of applying to the electrostatic image a developing powder in the form of a dispersion in an electrically insulating liquid. Although any of the other conventional finely divided solid materials used heretofore for the visual development of an electrostatic image may be the solid component of this liquid dispersion, carbon black is presently preferred. Accordingly, although carbon black will be referred to exclusively hereinafter and in the claims, it must be understood that the other conventional electrically attractable powders may be substituted for the carbon black;

Prior art methods for visual developing of electrostatic images include dry powder spraying, dusting, rolling with beads of a solid opaque developer, and the use of liquid mists. The dry powder method requires bulky equipment and many auxiliary devices for both charging the powder cloud and for applying the powder cloud to the electrostatic image to be developed. Another disadvantage of this method is the minimum size particle which can be readily suspended in air. This, of course, limits the resolution obtainable in the final image. Further disadvantages of the powder cloud technique are the relatively slow rate at which it can be accomplished and its lack of adaptability to a continuous process. Similar disadvantages are inherent in any developing method in which an air suspension is employed and thus apply equally to the use of a liquid mist, spray or fog of the sort disclosed in US. Patent 2,551,582 which is merely an air suspension of a liquid. The liquid development method of my invention eliminates the aforesaid disadvantages. The maximum particle size which can be adapted to my procedure is that of colloidal particles and for this reason all particles in the liquid suspension are well below the maximum resolution of a silver halide print. Furthermore, no electrostatic charging ofthe developer powder-liquid vehicle suspension is required in the practice of my invention, and this is an advantage over the prior art powder cloud techniques which require powder-charging apparatus.

An inherent disadvantage in both the liquid suspension technique of my invention and the dry powder cloud technique of the prior art is the halo effect which is exhibited to a noticeable extent. Halo may be described as a dark ring around a light area or a white ring around a dark area. The chief cause of this phenomenon is the field existing between adjacent charged and uncharged areas. The deposition of the developer powder in both the dry powder cloud procedure and the liquid suspension procedure is essentially a field phenomenon, with the cloud or the liquid respectively being utilized principally to bring the carbon particles close to the surface to be developed. At this surface, the electric field existing due to the charge configuration on the insulating material act upon the particles and cause them to be deposited in various areas; A halo results because of the fundamental electrical parameters of these differing field areas. This halo formation is minified by the use of a development electrode spaced close to the surface under development and tied electrically to the backing layer. This expedient, however, does not eliminate the halo effect. On the other hand, the roller development technique constituting a preferred embodiment of my invention completely eliminates the halo formation inas much .as the use of a roller brings the carbon particles directly and forcefully into contact with the surface to be developed and the electrostatic charge holds these particles at their deposited positions on the surface to be developed. No field is required to transfer the particles to the surface of the plate, and consequently the halo effect is eliminated. 1

To avoid the disadvantages inherent in the presently employed developing methods described above, I have found that application of finely divided particles of a powdered material such as charcoal, coal, colored glasses, resins or the like may be most advantageously effected by applying these particles in the form of a dispersion in a suitable dielectric liquid.

are suitable, although when a very high degree of resolution is desired, the optimum particlesize may be within the range of 0.01 to 0.5 micron in diameter.

The various powders may be dispersed in any liquid possessing a suitable electrical resistivity. Conventional insulating oils and other liquid hydrocarbons and chlorinated liquid hydrocarbonssuch as carbon tetrachloride, which boil at temperatures between 70 and 180 C. have been satisfactorily employed. I havealso found that inorganic liquids, such as high purity water, may also be used satisfactorily in practicing the invention. The powder, preferably carbon, after suitable drying and cleaning, is' added to the liquid and is milled in a ball mill for an extended period of time sufficientto reduce the carbon particle size to the desired maximum of 40 microns or less.

When an electrostatic image is contacted by a dispersion of this type, the carbon is deposited on the charged areas in a faithful reproduction of the original electrostatic image. This deposition of carbon has been found to beenhanced by placing a metal electrode spaced closely to the surface of the electrostatic image and flowing the liquid developer between these 'two surfaces.

The precipitation of the-carbon particles is thus accelerated and there results a very uniform continuous-tone image. This procedure is adaptable, in addition'to the aforementioned printing process, to all forms of electrophotography, and in particular I have foundthis liquid development procedure to be as highly satisfactory for line drawing applications as for continuous-tone work.

An alternative modification of the foregoing procedure which I have found to be adaptable to theaforementioned printing operation and also to other forms of electrostatic photography and image production is the coating of an insulating roller with the fine carbon liquid suspension and then moving the thus-coated roller over the electrostatic image on the surface of the photoconductive layer. The developer powder is thus transferred from the surface of the roller to the charged areas of the electrostatic image. When passing over uncharged areas of the image the carbon remains attached to the roller. and is not deposited on the uncharged portions of the surface carrying the electrostatic charge pattern. A flexible roller, advantageously of rubber, is preferred for this purpose as it conforms more faithfully to the surface. irregularities of the print. The roller must of course be covered uniformly with the developer solution layer. To accom plish this, the roller is coated with or immersed in this liquid developer comprising the aforementioned suspension of carbon black in an insulating organic liquid vehicle and is drained so as to leave a thin layer of the developer solution completely and uniformly distributed over the surface of the rubber roller. This layer is held on by surface tension and the roller may be left wet with the developer liquid or may be allowed to dry before it is rolled over the surface carrying the electrostatic image. I have found both procedures to work equally well. An outstanding advantage of this roller type development technique of my invention is its adaptability to a high speed repetitive printing process.

I claim:

1. The method of producing a visual reproduction of a visual object which comprises: providing a first assembly constituting a layer of electrically conductive material and a photoconductive insulating layer of material having a resistivity in the dark of at least 10 ohm-cm. and a resistivity when exposed to light of, not more than 10 ohm-cm, disposed thereon and secured thereto and a separate second assembly comprising a layer of electrically semiconductive material and a supporting metal layer secured thereto and lying along the layer of semiconductive material, the resistance across said layer of semiconductive material being between about 10" and 10 ohms, said semiconductive material being selected from materials which are less insulating that the material of the photoconductive insulating layer; disposing said assemblies in intimate relationship With the surface of the photoconductive insulating layer of the first assemblyin physical contact with the surface of the electrically semiconductive layer of the second assembly, applying to the supporting metal layer a DC. electric potential with respect to the layer of electrically conductive material whereby charge is leaked from the source of potential through the area of said physical contact to the photoconductive insulating layer and projecting a light image of said visual object onto the photoconductive insulating layer thus establishing on the layer of photoconductive insulating material an electrostatic reproduction of said light image, separating the portion'of thefirst assembly bearing the electrostatic reproduction of the object to be reproduced from said second assembly and applying a dispersion of charged'particles of powdered material to the surface of the photoconductive insulating material bearing the electrostatic reproduction of the image with the resultant formation on the said surface of a powder deposit comprising a visual reproduction of said light *image.

semiconductive material being between about and 10 ohms, said semiconductive material being selected from materials which are less insulating than the mate- 'rial'o'f the photoconductive insulatinglayer; disposing said assemblies in intimate relationship With the surface of the photoconductive insulating layer of the first assembly in physical contact with the surface of the electrically semiconductive layer of the second assembly, applying to the supporting metal layer a DC. electric potential with respect to the layer of electrically conductive material whereby charge is leaked from the source of potential through the area ofsaid physical contact to the photoconductive insulating layer and projecting a light image of said visual object onto the photoconductive insulating layer thus establishing on the layer of photoconductive insulating material an electrostatic reproduction of said light image, separating the portion of the first assembly bearing the electrostatic reproduction of the object to be reproduced from said second assembly and applying a dispersion of charged particle-s of powdered material to athesur'face of the photoconductive insulating material bearing the electrostatic reproduction of the image with the resultant formation on the said surface of a powder deposit comprising a visual reproduction of said light image and transferring the visual powder reproduction of the light image to a permanent base sheet.

3. The method of producing a visual reproduction of a visual object which comprises: providing a first assembly constituting a layer of electrically conductive material and a photoconductive insulating layer of material 'liaving' a resistivity in the dark of at least 10 ohm-cm. and a resistivity when exposed to light of not more than 10 ohm-'cm., disposed thereon and secured thereto and a separate second assembly comprising a layer of electrically semiconductive material and a supporting metal layer secured thereto and-lying alongthe layer of semiconductive material, the resistance across said layer of semiconductive material being between about 10 and 10 ohms, said semiconductive material being selected from materials which are less insulating than the material of the photoconductive insulating layer; disposing.

said assemblies in intimate relationship with the surface of the vphotoconductive insulating layer of the first assembly invphysical contact with the surface of the electrically semiconductive layer of the second assembly, applying to the supporting metal layer a DC. electric potential with respect toithe layer of electrically conductive material whereby charge isileakedfrom the source of potential through the area of said physical contact to the photoconductive insulating layer and projecting a light image of "said visual object onto the photoconductive insulating layer thus establishing on the layer of photoconductive insulatingmaterial an electrostatic reproduction of said light image, separating the portion of the first assembly bearing the electrostatic reproduction of the object to be reproduced from said second assembly and applying a dispersion of charged particles of carbon in a dielectric liquid to :the surface of the photoconductive insulating material bearing the electrostatic reproduction of the image with the resultant formation on the said surface of a carbon deposit comprising a visual reproduction of said light. image, said carbon particles being dispersed in a :dielectric liquid of the-group consisting of insulating oils, hydrocarbons and chlorinated hydrocarbons and possessed of a boiling point between 70 C. and 180 'C., and transferring the visual'carbon powder reproduction of the light image to a permanent base sheet. I .4. The method of producing a visual object which 12 comprises: providing a first assembly constituting a layer of-electrically conductive material and a photoconductive insulating layer of material having a resistivity in the dark of at least 10 ohm-cm. and a resistivity when exposed to light of not more than 10 ohm-cm, disposed thereon and secured thereto and a separate second assembly comprising a layer of electrically semiconductive material and a supporting metal layer secured thereto and lying along the layer of semiconductive material, the resistance across said layer of semiconductive material being between about 10 and 10 ohms, said semiconductive material being selected from materials which are less insulating than the material of the photoconductive insulating layer;,applying to the supporting metal layer a DC electric potential with respect to the layer of electrically conductive material, projecting a light image of said visual object onto a given portion of the surface of said photoconductive insulating layer, and simultaneously with said projection of said light image, disposing said assemblies in intimate relationship so that said given portion of the surface of the photoconductive insulating layer of the first assembly is in physical contact with'the surface of the electrically semiconductive layer of the second assembly, whereby charge is leaked from the source of potential through the area of said physical contact to the photoconductive insulating layer and thus establishing on the layer of photoconductive insulating material an electrostatic reproduction of said light image, separating the portion of the first'assembly bearing the electrostatic reproduction of the object to be reproduced from said second assembly and applying a dispersion of charged particles of powdered carbon to the surface of the photoconductive insulating material bearing the electrostatic reproduction of the image with the resultant formation on the said surface of a carbon deposit comprising a visual reproduction of said light image and transferring the visual carbon powder reproduction of the light image to a permanent base sheet.

5. The method of producing a visual reproduction of a visual object which comprises: providing a first assembly constituting a layer of electrically conductive mate rial and a photoconductive insulating layer of material having a resistivity in the dark of at least 10 ohm-cm. and a resistivity when exposed to light of not more than 10 ohm-cm, disposed thereon and secured thereto and a separate second assembly comprising a layer of electrically semiconductive material and a supporting metal layer secured thereto and lying along the layer of semiconductive material, the resistance across said layer of semiconductive material being between about 10 and 10 ohms, said semiconductive material being selected from materials which are less insulating than the material of the photoconductive insulating layer; applying to the supporting metal layer a DC. electric potential with respect to the layer of electrically conductive material, projecting a light image of said visual object onto a given portion of the surface of said photoconductive insulating layer and immediately thereafter and not longer than a time interval which is small compared to the lifetime of the carriers developed by the light projected onto the photoconductive layer, disposing said assemblies in intimate relationship so that said given portion of the surface of photoconductive insulating layer of the'first assembly is in physical contact with the surface of the electrically semiconductive layer of the second assembly, whereby charge is leaked from the source of potential through the area of said physical contact to the photoconductive insulating layer and thus establishing onthe layer of photoconductive insulating material an electrostatic reproduction of said light image, separating the portion of the first assembly bearing the electrostatic reproduction of the object to be reproduced from said second assembly and applying a dispersion of'charged particles of powdered carbon to the surface of the photoconductive insulating material bearing'the electrostatic reaeaaesa production of the image with the resultant formation on the said surface of a carbon deposit comprising a visual reproduction of said light image and transferring the visual carbon powder reproduction of the light image to a permanent base sheet.

6. The method of producing a visual reproduction of a visual object which comprises: providing a first assembly constituting a layer of electrically conductive material and a photoconductive insulating layer of material having a resistivity in the dark of at least ohm-cm. and a resistivity when exposed to light of not more than 10 ohm-cm., secured thereto and disposed thereon and a second assembly comprising a layer of electrically semiconductive material and a supporting metal layer secured thereto and lying along the layer of semiconductive material, the resistance across said layer of semiconductive material being between about 10" and 10 ohms, said semiconductive material being selected from materials which are less insulating than the material of the photoconductive insulating layer; disposing said as semblies in intimate contact with a portion of the photoconductive insulating layer adjacent the electrically semiconductive layer, applying to the supporting metal layer a DC. electric potential with respect to the transparent layer of electrically conductive material whereby charge is leaked from the potential through the area of said intimate contact to the photoconductive insulating layer and thus establishing on the layer of photoconductive insulating material a uniform electrostatic charge, separating the portion of the first assembly bearing said uniform charge from said second assembly and projecting a light image of said visual object onto said given portion of the photoconductive insulating layer bearing said uniform electrostatic charge whereby a charge pattern is produced on the photoconductive insulating layer corresponding to the visual image projected thereon and applying a dispersion of charged particles of powdered carbon to the surface of the photoconductive insulating material hearing the electrostatic reproduction of the image with the resultant formation on the said surface of a carbon deposit comprising a visual reproduction of said light image.

7. An electrophotographic device comprising a photoconductive element comprising a layer of electrically conductive material and a layer of photoconductive insulating material having a resistivity when exposed to light of not more than 10 ohm-cm, contiguous thereto, a charge applying element comprising a layer of electrically conductive material and a layer of electrically semiconductive material contiguous thereto, the two elements being disposed with the surface of the layer of photoconductive material of the one element lying along and in intimate contact with the surface of the semiconductive material of the other element, the resistance from a point on the surface of the semiconductive material to a corresponding point on its supporting layer of electrically conductive material lying between about 10" and 10 ohms, a source of electrical potential, connections between said source of electrical potential and the conductive layers of each of said elements for impressing the potential across the four layers of the two elements, means for directing a light image of an object toward the layer of photoconductive material to be recorded thereon, and shield means for restricting illumination to a narrow predetermined area of said layer of photoconductive material.

8. An electrophotographic device comprising a photoconductive element comprising a layer of electrically conductive material and a layer of photoconductive insulating material having a resistivity when exposed to light of not more than 10 ohm-cm; contiguous thereto, a charge applying element comprising a layer of electrically conductive material and a layer of electrically semiconductive material contiguous thereto, the two elements being disposed with the surface of the layer of photoconductive material of the one element lying along and in intimate contact with the surface of the semiconductive material of the other element, resistance from a point on the surface of the semiconductive material to its supporting layer of conductive material being of substantially 10 to 10 ohms, a source of electrical potential, connections between said source of electrical potential and the conductive layers of each of said elements for impressing the potential across the four layers of the two elements and means for directing a light image of an object toward the layer of photoconductive material to be recorded thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,277,013 Carlson Mar. 17, 1942 2,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 1944- 2,551,582 Carlson May 8, 1951 2,558,900 Hooper July 3, 1951 2,600,580 Sabel et a1. June 17, 1952 2,618,551 Walkup Nov. 18, 1952 2,659,670 Copley Nov. 17, 1953 2,693,416 Butterfield Nov. 2, 1954 2,726,940 Buhler Dec. 13, 1955 2,735,784 Grieg et al Feb. 21, 1956 2,774,921 Walkup Dec. 18, 1956 2,784,109 Walkup Mar. 5, 1957 2,807,233 Fitch et al. Sept. 24, 1957 2,825,814 Walkup Mar. 4, 1958 2,833,648 Walkup May 6, 1958 2,833,930 Walkup May 6, 1958 2,845,348 Kallman July 29, 1958 2,853,383 Keck Sept. 23, 1958 2,904,431 Moncrieff-Yeates Sept. 15, 1959 OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pages 12-22. 

1. THE METHOD OF PRODUCING A VISUAL REPRODUCTION OF A VISUAL OBJECT WHICH COMPRISES: PROVIDING A FIRST ASSEMBLY CONSTITUTING A LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL AND A PHOTOCONDUCTIVE INSULATING LAYER OF MATERIAL HAVING A RESISTIVITY IN THE DARK OF AT LEAST 1012 OHM-CM. AND A RESISTIVITY WHEN EXPOSED TO LIGHT OF NOT MORE THAN 1010 OHM-C., DISPOSED THEREON AND SECURED THERETO AND A SEPARATE SECOND ASSEMBLY COMPRISING A LAYER OF ELECTRICALLY SEMICONDUCTIVE MATERIAL AND A SUPPORTING METAL LAYER SECURED THERETO AND LYING ALONG THE LAYER OF SEMICONDUCTIVE MATERIAL, THE RESISTANCE ACROSS SAID LAYER OF SEMICONDUCTIVE MATERIAL BEING BETWEEN ABOUT 107 AND 109 OHMS, SAID SEMICONDUCTIVE MATERIAL BEING SELECTED FROM MATERIALS WHICH ARE LESS INSULATING THAT THE MATERIAL OF THE PHOTOCONDUCTIVE INSULATING LAYER: DISPOSING SAID ASSEMBLIES IN INTIMATE RELATIONSHIP WITH THE SURFACE OF THE PHOTOCONDUCTIVE INSULATING LAYER OF THE FIRST ASSEMBLY IN PHYSICAL CONTACT WITH THE SURFACE OF THE ELECTRICALLY SEMICONDUCTIVE LAYER OF THE SECOND ASSEMBLY, APPLYING TO THE SUPPORTING METAL LAYER OF A D.C. ELECTRIC POTENTIAL WITH RESPECT TO THE LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL WHEREBY CHARGE IS LEAKED FROM THE SOURCE OF POTENTIAL THROUGH THE AREA OF SAID PHYSICAL CONTACT TO THE PHOTOCONDUCTIVE INSULATING LAYER AND PROJECTING A LIGHT IMAGE OF SAID VISUAL OBJECT ONTO THE PHOTOCONDUCTIVE INSULATING LAYER THUS ESTABLISHING ON THE LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL AN ELECTROSTATIC REPRODUCTION OF SAID LIGHT IMAGE, SEPARATING THE PORTION OF THE FIRST ASSEMBLY BEARING THE ELECTROSTATIC REPRODUCTION OF THE OBJECT TO BE REPRODUCED FROM SAID SECOND ASSEMBLY AND APPLYING A DISPERSION OF CHARGED PARTICLES OF POWDERED MATERIAL TO THE SURFACE OF THE PHOTOCONDUCTIVE INSULATING MATERIAL BEARING THE ELECTROSTATIC REPRODUCTION OF THE IMAGE WITH THE RESULTANT FORMATION ON THE SAID SURFACE OF A POWDER DEPOSIT COMPRISING A VISUAL REPRODUCTION OF SAID LIGHT IMAGE. 